I got Terraria over half a year ago, actually, but I haven't had much time to play it. Over the Winter Holidays, I plan to fix that. The farthest I've gotten so far is iron/copper armor. Any tips for a newbie?

P.S. I already know about most game mechanics: NPCs, Potions, Bosses, etc.

When you get to the Goblin Tinkerer, don't bother reforging unless it's a really bad modifier on a weapon. Wait until you get an abundance of money then try it out, but the whole thing is a huge gamble and you can very easily lose tens of gold from just a few shots.

Eater of Worlds can be very easily farmed to make a LOT of money. Making several summoners (Worm Food, 30 Vile Powder and 15 Rotten Chunks, Vile Powder made from Vile Mushrooms at an Alchemy Station, Rotten Chunks found on any Eater of Souls, and the actual summoner made at a Demon Altar) and fighting him once you get past the Dungeon or Jungle can be a snap, and all the materials he drops together can be sold for a total of a little over 10 gold each. So multiply 10 by however many summoners you made and you got a lot of mula.

Dig down far enough and you'll find the Underworld! The Underworld is the hardest pre-hardmode (I'll get to that later) biome in the game, so don't enter it until you KNOW, not THINK, you're ready. The Underworld gives the best pre-hardmode material in the game, Hellstone. The best material comes at a cost though, it burns you unless you have an Obsidian Skull, Obsidian Shield or Obsidian Horseshoe, so be careful! It also spawns a little bit of lava every time you mine it. The Underworld is also home to the Wall of Flesh!

The Wall of Flesh is the trigger for Hardmode. Once you beat him, no turning back. You cannot turn off Hardmode. The Wall of Flesh can be spawned by throwing a Guide Voodoo Doll into lava (which kills the Guide, but no worry, another one will spawn soon after). Guide Voodoo Dolls can be found on a special Demon called a Voodoo Demon which always drops a doll. You can tell a Voodoo Demon from a regular Demon by looking at his talons; if he's got a Guide-esque doll hanging from there, you found yourself a Voodoo Demon. Also be mindful that killing him over lava will result in a very, very, very bad time if you didn't catch the doll and you're unprepared. Anyhow, back to the Wall of Flesh. He can be pretty easily killed by making a floor across the entire Underworld (don't worry about materials, you'll have a huge amount of stacks of dirt and stone by the time you get to the Underworld) and running across it when you spawn him. He moves across the map and tries to kill you. He has eyes and a mouth, these are his weak points, shooting anywhere else on the Wall will NOT damage him. The Wall sends out The Hungry to deal with you which fly off once you hit them and home in on you, sort of like Demon Eyes. Once you get him to below half health, he'll spit out Leeches to kill you that act like worms. They do a surprising amount of damage, so watch out for these guys. The Wall of Flesh drops very good loot, so killing him even after Hardmode is never a bad idea.

Hardmode is the hardest point of Terraria. It will essentially take up the second half of the game. All monsters are a LOT stronger, and a new biome is spawned: The Hallow. The Hallow is sort of the pretty princess love and sparkles land, but everything will still try to kill you there. Both the Hallow and the Corruption spread extremely fast now in Hardmode, so keep watch! (Note, there ARE ways to prevent this, such as digging 4 block wide trenches around your house to the Underworld so the Corruption and Hallow won't reach you. However, destroying Demon Altars [we'll get to that later] spawns a new area of Hallow or Corruption somewhere on the map, so keep watch that that might end up to be right under your house!) The Wall of Flesh always drops a Pwnhammer, which is the only hammer in the game capable of destroying Demon Altars, aka "cleansing the world of the evil". Demon Altars, upon being smashed into tiny, tiny bits, will spawn more and more Hardmode ore into your world. The first ore is Cobalt. This is the weakest one. The next is Mythril. This is the medium one. The last is Adamantite. This is the absolute toughest material in the game. Smashing Demon Altars will spawn it in that order and repeat upon smashing more, so after you activate Hardmode smash EVERY SINGLE DEMON ALTAR YOU CAN EVER FIND. However be wary of how it also spawns a dot of Hallow or Corruption in the world upon destroying it, so keep watch of that. Cobalt requires a Hellstone Pickaxe to mine, Mythril requires a Cobalt Drill to mine, and Adamantite requires a Mythril Drill to mine.

Phew. Okay. That's enough typing for now. If you have any more questions just ask me.

Holding shift down over a block with automatically select the tool needed to break it. Shift over open air will hold a torch out, and over water will hold a glowstick out. That can save a lot of time and speed things up.
'E' is your grappling hook. 'T' is to throw things (careful with that, you might lose a good weapon or something otherwise lol)

Assign the Nurse to the same room as you respawn in, for quick healing purposes.

Dig a 2x2 pit in front of your doors to keep you safe from pre-hardmode bloodmoons. (Zombies will fall in the pit and have to jump to reach your doors. They cannot jump and open doors at the same time).

Try building outposts when exploring new places like the jungle or dungeon, and setting your spawn point in them. It can be nice to respawn right next to the dungeon instead of having to travel all the way back there if you die.

I just got the game few days ago, so far the wiki has helped me a lot, so start with that ?. You call yourself a newbie and you say you know about the Potions, bosses and all that :P, I wouldn't call that newbie level :P. Any ways have a look at that and see if that helps :)

Demon alters. Break all the demon alters with a hammer. Just kidding, but do find the alter that's close to your house and make an underground path to it. Also, start your helivater as soon as possible. Make it near the center to avoid as much of the hard mode V as you can. Finally, start making your house a safe place.